The invention relates to a diagnostic imaging system, in particular a magnetic resonance imaging system which is provided with an ultrasound probe.
Such a magnetic resonance imaging system is known from the U.S. Pat. No. 5,146,924.
The known magnetic resonance imaging system comprises a receiver antenna for receiving magnetic resonance signals which is mounted in an ultrasound transducer. The ultrasound transducer includes an ultrasound source for generating ultrasound waves in the object to be examined, such as a patient who is to be examined. The ultrasound transducer also includes the ultrasound probe. The ultrasound probe detects ultrasound echoes, i.e. ultrasound waves which are reflected in an object to be examined. The central processor of the known magnetic resonance imaging system derives from the detected ultrasound waves an ultrasound image of a part of the object to be examined. From this ultrasound image the user determines the position of the part of the anatomy to be examined, notably the organ of interest. Subsequently, on the basis of the position of the part to be examined as detected from the ultrasound image, there are excited (nuclear) spins in the object to be examined of the part to be examined upon which excitation magnetic resonance signals are generated and a magnetic resonance image of the part to be examined is reconstructed from the magnetic resonance signals. The known magnetic resonance system employs the xe2x80x98Overhauserxe2x80x99-effect to generate the magnetic resonance image. The known magnetic resonance imaging system employs the ultrasound image only for the determination of the position of the part of the object of which a magnetic resonance image is made.
An object of the invention is to provide a diagnostic system which is suitable to supply diagnostic image with a higher diagnostic quality, notably having a higher diagnostic information content.
This object is achieved by the diagnostic imaging system which, according to the invention comprises a reconstruction unit for reconstructing a diagnostic image from the magnetic resonance signals and the ultrasound echoes.
The magnetic resonance signals are generated by RF-excitation of (unclear) spins in the object which is placed in a stationary magnetic field. Temporary magnetic gradient fields are applied so that the Larmor frequency of the excited spins is made spatial position dependent. Thus, spatial encoding of the magnetic resonance signals by the frequencies and phases of the magnetic resonance signals is achieved.
The diagnostic image combines image information both from the magnetic resonance signals and from the ultrasound echoes. For example the diagnostic image includes a portion of pixels that are derived from the magnetic resonance signals and another portion of pixels that are derived from the ultrasound echoes. The information included in the magnetic resonance signals has a high spatial resolution but a low temporal resolution. On the other hand, the information included in the ultrasound echoes has a lower spatial resolution but a much higher temporal resolution. In particular the information in the magnetic resonance signals is time-averaged over a range of about 50 ms to about 0.5 s and the information in the ultrasound echoes pertains to short periods of time of about 5-20 ms. The information in the magnetic resonance signals may have a high spatial resolution in that details as small as 0.5 mm are faithfully represented in the magnetic resonance image. The diagnostic image thus combines image information with a high spatial resolution with image information with a high temporal resolution. Thus, the diagnostic image shows in particular image information of high temporal resolution of rapidly moving parts in the patient to be examined while the anatomical surroundings of the rapidly moving portions are accurately displayed with a high spatial resolution. For example, moving portions of the patient""s heart are displayed against the correctly spatially highly resolved anatomical background.
In another example, the diagnostic image combines functional information from the ultrasound echoes of the reconstruction which is displayed in the anatomical information from the magnetic resonance signals. For example, in the diagnostic image colour-Doppler values derived from the ultrasound echoes may replace grey-values derived from the magnetic resonance signals.
These and other aspects of the invention will be elaborated with respect to the preferred embodiments as defined in the dependent Claims.
In a preferred embodiment the reconstruction unit is arranged to derive a magnetic resonance image and a preliminary ultrasound image from the magnetic resonance signals and the ultrasound echoes, respectively. The diagnostic image may be derived from the magnetic resonance image and the preliminary ultrasound image. The term xe2x80x98preliminary ultrasound imagexe2x80x99 in this application indicates any ultrasound image to which correction on the basis of the magnetic resonance image or registration relative to the magnetic resonance image is still to be performed. For example, respective portions of the preliminary ultrasound image and of the magnetic resonance image are included into the diagnostic image. In an other example, the diagnostic image is formed in that at least respective portions of the magnetic resonance image and the preliminary ultrasound image are displayed alternatingly.
In a further preferred embodiment, the magnetic resonance image and the preliminary ultrasound image are registered in a common co-ordinate system. That is, between respective positions in the magnetic resonance image and in the preliminary ultrasound image the geometric relation is established. On the basis of this geometric relation positions in the magnetic resonance image and in the ultrasound image are registered in correspondence with the geometric relation between the imaged positions in the object. This may be achieved by relating positions in the preliminary ultrasound image an in the magnetic resonance image to a common reference frame. Such a common reference system is for example defined in the examination room in which the diagnostic imaging system is set up. Notably, a position detection system is provided in the examination room. The position detection system measures the positions of the ultrasound probe relative to the patient to be examined. The position detection system also determines the geometric relation between positions in the patient to corresponding positions in the magnetic resonance image. The position detection system may include an optical or acoustical position detection system which measures the positions of the patient and the ultrasound probe. The position of the ultrasound probe determines a region, in particular a slice, of the patient from which the ultrasound echoes are received. Thus, from the measurement of the position of the ultrasound probe, the part imaged in the preliminary ultrasound image is established. Notably, when an optical position detection system is employed, the ultrasound probe and the patient are fitted with light-emitting diodes (LEDs) or infrared-emitting diodes (IREDs). The radiation, notably light or infrared radiation, from the LEDs or IREDs is detected from two or more directions by means of a camera-unit. The camera-unit picks up images from the sets of LEDs or IREDs. The diagnostic imaging system comprises a computer which is also programmed to derive the position of the ultrasound probe relative to the patient from the image picked-up by the camera-unit of the LEDs or IREDs. The gantry of the magnetic resonance imaging system is preferably also fitted with LEDs or IREDs and the camera-unit further picks-up images of the MR-gantry, notably these images also feature images of the LEDs or IREDs on the MR-gantry. The computer is also arranged to calculate the position of the gantry relative to the patient to be examined. Further, the computer is arranged to compute the geometric relation between positions in the patient and the corresponding positions in the magnetic resonance image on the basis of the measured relative position of the patient to the MR-gantry and on the basis of the applied temporary magnetic gradient fields, such as the slice selection, phase encoding and read-out gradients. Thus it is achieved to establish the geometric relationship between corresponding positions in the magnetic resonance image and in the preliminary ultrasound image. The magnetic resonance image and the preliminary ultrasound image are registered in a common co-ordinate system which is based on the geometric relation between the magnetic resonance image and the preliminary ultrasound image. In this respect, corresponding positions in either image pertain their common position in the patient. As the magnetic resonance image and the preliminary ultrasound image are registered in the common co-ordinate system the diagnostic image which is derived from the magnetic resonance image and the preliminary ultrasound image shows image information from the magnetic resonance image and from the preliminary ultrasound image in their correct mutual geometric relation. That is, e.g. respective portions of the preliminary ultrasound image and of the magnetic resonance image are included in the diagnostic image at correct relative positions to one another.
In a preferred embodiment of the diagnostic system according to the invention the magnetic resonance image is employed to correct the preliminary ultrasound image and form the diagnostic image as the corrected ultrasound image. The diagnostic image may also be formed by combining portions if the corrected ultrasound image and the magnetic resonance image. Notably, the magnetic resonance image can be used to correct geometric distortions in the preliminary ultrasound image. For example, portions in the ultrasound image relating different tissue types are distinguished on the basis of the magnetic resonance image. For example, local ultrasound sound velocities for such different tissue types are derived from the magnetic resonance image and the preliminary ultrasound image is corrected for distortions due to differences between ultrasound sound velocities. Further, tissue interfaces can be located in the magnetic resonance image and related to US-echoes from these tissue interfaces so that renditions of tissue interfaces in the magnetic resonance image and in the corrected ultrasound image are in correspondence.
In a preferred more simple embodiment of the diagnostic imaging system corresponding anatomical landmarks are identified in the magnetic resonance image and in the preliminary ultrasound image. From the respective positions of the anatomical landmarks in the magnetic resonance image and correspondingly in the preliminary ultrasound image the correct geometric relation between corresponding positions in the preliminary ultrasound image and in the magnetic resonance image are calculated. On the basis of the geometric relation based on the corresponding anatomical landmarks, the preliminary ultrasound image and the magnetic resonance image are registered in the common co-ordinate system. From these registered preliminary ultrasound image and the registered magnetic resonance image the diagnostic image is formed.
In another embodiment of the diagnostic image according to the invention the position of the ultrasound probe is measured on the basis of the magnetic resonance signals. Notably, at least a part of the magnetic resonance signals picked-up by the receiver antenna relates to or originates from the ultrasound probe, e.g. the ultrasound probe or markers having a substantial magnetic susceptibility being fitted to the ultrasound probe are also imaged in the magnetic resonance image. Thus, from the position of the ultrasound probe as measured on the basis of the magnetic resonance signals the geometric relation between the ultrasound probe and thus the preliminary ultrasound image and the magnetic resonance image is obtained. On the basis of this geometric relation the preliminary ultrasound image and the magnetic resonance image can be registered in the common co-ordinate system and the diagnostic image formed on the basis of the registered preliminary ultrasound image and the registered magnetic resonance image.
In another embodiment the ultrasound probe is fitted with micro-coils. While the magnetic resonance signals are generated the micro-coils pick-up a part of the magnetic resonance signals form the close neighbourhood of the micro-coils. Thus the magnetic resonance signals picked-up by the micro-coils represent the position of the ultrasound probe. The micro-coils produce electrical (induction) signals in response to the magnetic resonance signals. These electrical signals in turn represent the position of the ultrasound probe relative to the patient and relative to the MR-gantry. These electrical signals from the micro-coils are advantageously employed to determine the geometric relation between the preliminary ultrasound image derived from the ultrasound echoes picked-up by the ultrasound probe and the magnetic resonance image.
The functions of the diagnostic imaging system are in practice performed under the control of a computer programme including various instructions which enable the diagnostic imaging system to produce the technical effects involved in the present invention. Such a computer programme is loaded into e.g. the working memory or accessible to the processor of e.g. a control unit and/or a reconstruction unit or combination unit of the diagnostic imaging system. The computer programme may be made available on a data carrier such as a CD-ROM disk, or the computer programme may be downloaded from a network, such as the world-wide web.
Another object of the invention is to provide a magnetic resonance imaging system which also enables the acquisition of information which relates to current instants or very short periods of time.
This further object is achieved by means of a magnetic resonance imaging system according to the invention which is provided with a display system for the combined display of information contained in the magnetic resonance signals and information contained in the ultrasound waves.
The display system includes inter alia a signal processing unit and a monitor. An LCD (liquid crystal) monitor is suitable for use in combination with the magnetic resonance imaging system, because such an LCD monitor is not very sensitive to the magnetic (temporary) gradient fields necessary to generate and receive the magnetic resonance signals. The LCD monitor is preferably electromagnetically shielded from the magnetic resonance imaging system in order to prevent the electronic signals controlling the LCD monitor from disturbing the acquisition of the magnetic resonance signals and to prevent the magnetic resonance signals from disturbing the control of the LCD monitor. The signal processing unit is arranged to reconstruct a magnetic resonance image from the magnetic resonance signals and also to form an ultrasound image on the basis of the detected ultrasound waves. The detected ultrasound waves have notably been reflected as ultrasound echoes in the object to be examined. The magnetic resonance signals are detected by means of the receiver antenna, such as a receiver coil, so as to be applied to the signal processing unit. The ultrasound waves are detected by the ultrasound probe. The ultrasound probe produces US detection signals in response to the detected ultrasound waves. The US detection signals represent the information contained in the ultrasound waves; for example, the signal levels of the US detection signals correspond to the strengths of the ultrasound waves. The US detection signals are also applied to the signal processing unit. The magnetic resonance image and the ultrasound image are both represented by image signals in the signal processing unit, for example, electronic video signals. These image signals are applied to the monitor so that the image information contained in the magnetic resonance signals and in the ultrasound waves is visualized.
In conformity with the invention the display system displays the magnetic resonance image and the ultrasound image in combined form on the monitor. The ultrasound image contains mainly instantaneous information, or at least information relating to very short periods of time, i.e. periods of time shorter than the time typically required for the acquisition of the magnetic resonance signals for the magnetic resonance image; these short periods of time notably have a duration of from approximately 5 to 20 ms. The period of time over which averaging effectively takes place during the formation of the magnetic resonance image is dependent on the spiral resolution of the magnetic resonance image and on the exact acquisition strategy used to sample the k space. It has been found in practice that the period of time over which the information in the magnetic resonance image is averaged amounts to from approximately 50 ms to some tenths of a second. Combined display of the ultrasound image and the magnetic resonance image provides information concerning instantaneous events in the object to be examined, for example a patient to be examined, together with the time averaged, but spatially suitably resolved information. This is because, generally speaking, the magnetic resonance image has a spatial resolution which is higher than that of the ultrasound image. Due to the combined display, the information of high spatial resolution from the magnetic resonance image can b combined with information of high temporal resolution from the ultrasound image. The magnetic resonance image contains information which has been averaged in time to a given extent. Due to the combined display of the ultrasound image and the magnetic resonance image, information concerning instantaneous events in the object to be examined, for example a patient to be examined, can be made available together with time averaged information.
The ultrasound image contains, for example, mainly functional information concerning physical processes taking place in the object to be examined. For example, quantities which quantify the flow of a liquid, for example, blood through the blood vessels of the patient to be examined, are concerned. The magnetic resonance imaging system according to the invention enables the reproduction of such functional information contained in the ultrasound image in, for example, the magnetic resonance image. The magnetic resonance image preferably reproduces with a high spatial resolution the anatomical structure of the patient to be examined. The radiologist is thus offered a good quantitative image of the functional information and also a suitably spatially resolved image of the region wherefrom the functional information originates.
The combined display of the magnetic resonance image and the ultrasound image can be realised in various ways. For example, there is formed a composite image with partly brightness values from the magnetic resonance image and partly brightness from the ultrasound image. It is alternatively possible to alternate the magnetic resonance image and the ultrasound image. For example, the magnetic resonance image and the ultrasound image alternate at a rate of approximately 20 fps (frames per second). Furthermore, it is also possible to superpose the ultrasound image as an xe2x80x9coverlayxe2x80x9d on the magnetic resonance image.
For example, grey values in the magnetic resonance image are replaced by colour Doppler values from the ultrasound image.
The magnetic resonance image as well as the ultrasound image can represent the distribution of the values of a physical quantity. For example, the physical quantity concerns perfusion or flow of a fluid of physiological importance. For example, cerebrospinal fluid (CSF) or arterial or venous blood are concerned. According to the invention the spatial distribution of physical quantities derived from physical quantities represented by the magnetic resonance image and by the ultrasound image can be reproduced in the composite image. For example, the combined image can reproduce the spatial distribution of the ratio of peak flow values and local mean flow values.
The reconstruction unit is preferably arranged to register the magnetic resonance image and the ultrasound image in a common reference system. For example, this can be realised by identification of corresponding anatomical details in the magnetic resonance image and in the ultrasound image. It is also possible to measure the position of the ultrasound probe by means of the magnetic resonance imaging system; to this end, the ultrasound probe is preferably provided with one or more identification members which are susceptible to the RF excitation. For example, microcoils are suitable identification members. Due to the RF excitations, the microcoils receive magnetic resonance signals which represent the position of the microcoils and hence of the ultrasound probe. The magnetic resonance image and the ultrasound image are registered in the common reference system on the basis of the measured position of the ultrasound probe. On the basis of the temporary (for example, read-out and phase encoding) gradient fields the magnetic resonance imaging system determines the position of the ultrasound probe as well as the position of the part of the object being imaged in the magnetic resonance image.
It is noted that advantageously according to the invention the correction of the (preliminary) ultrasound image and the registration of the (preliminary) ultrasound image may also be employed to correct and/or mutually register highly spatially resolved information of the magnetic resonance image and highly temporally resolved image information in the (preliminary) ultrasound image.